1. Field of the Invention
The present invention is related to the field of production logging of oil and gas wells. More specifically, the present invention is related to methods of determining volumetric flow rates of gas and liquid in highly inclined wellbores particularly when the flow regime substantially consists of stratified gas-liquid flow.
2. Description of the Related Art
It is desirable for the operator of an oil and gas well to be able to determine, with respect to depth in the wellbore, flow rates of various fluids flowing within the wellbore. The fluids typically include oil, gas and water. The wellbore operator uses knowledge of the flow rates of the fluids to determine, among other things, which subsurface earth formations introduce the fluids into the wellbore which are desired to be produced, typically the gas and/or oil, and which formations introduce fluid, typically water, which interferes with the intended fluid production of the wellbore.
Production logging of oil and gas wells is used to determine, with respect to depth, the flow rates of the various fluids flowing within the wellbore. Production logging includes lowering instruments into the wellbore at one end of an armored electrical cable. The instruments communicate signals along the cable to a recording system at the earth's surface wherein the instrument signals are converted into measurements corresponding to the flow rates of the fluids in the wellbore with respect to dept. The instruments typically include a device which measures fluid pressure, a device which measures specific gravity of the fluid in the wellbore, another device which is used to determine the fractional volume of water in the wellbore, and another device selected from a number of different types of such device which measures the velocity of the fluid flowing in the wellbore. As is understood by those skilled in the art, production logging instruments typically measure properties of the fluids in the wellbore only within a very small fraction of the total cross-sectional area of the wellbore. In vertical wellbores this limitation of production logging instruments typically does not pose any difficulty in determining volumetric flow rates of the individual fluids because fluid flow is typically sufficiently homogeneous to enable the response of the production logging instrument to reflect the bulk composition of the fluid. However, in highly inclined or horizontal wellbores, the fluids in the wellbore can segregate by gravity so that the less dense fluids fill the upper portion of the cross-sectional area of the wellbore and more dense fluids fill the lower portions of the cross-section of the wellbore. Gravity segregation may cause the production logging instruments to make measurements corresponding to the density, velocity, and fractional volume of only one of the fluids, depending on the fraction of the cross-section of the wellbore filled with each individual fluid, and on the position of the production logging instrument within the cross-section of the wellbore.
A particularly difficult flow condition to measure in highly inclined wellbores using the production logging tools known in the art is so-called stratified flow, in which the fluids in the wellbore comprise gas and liquid. Gas typically will occupy the upper portion of the wellbore, and the liquid will occupy the lower portion. In order for the wellbore operator to determine the volumetric flow rates of gas and liquid flowing in such a wellbore, he must be able to determine gas velocity, liquid velocity and the fractional amount of the wellbore cross-section occupied by each. Because the production logging tools known in the art measure fluid properties at only a small fraction of the total cross-sectional area of the wellbore, it is difficult to determine the fraction of the cross-sectional area and the velocities of gas and liquid using production logging instruments and methods known in the art.
Accordingly, it is an object of the present invention to provide a method of determining flow rates of stratified gas and liquid in a highly inclined wellbore in which it is not necessary to measure all three parameters: gas velocity, liquid velocity and fractional volume of each within the wellbore.